Lessons learned meanwhile with other plugins, so the code applied here has
actually been tested and proven to work. This reverts [8409] effectively,
but let's keep proper Babel and Trac versions for i18n explicitly as an extra.

After these changes the plugin finally loads cleanly in Trac 0.11 again.

Actually ChoiceOption appeared even after Trac 0.12 release, so this would
belong into a dedicated 0.13 branch, that we may have in the future.

I plan to rollback and convert any 0.12 stuff to regain backwards-compatibility
with Trac down to 0.11, and move development to 0.13 instead. This involves
mostly i18n stuff (could be done in a more compatible fashion too) and
especially the db connection handling, that has seen may changes.
It seems to have settled only lately in Trac 0.13dev .

exception_to_unicode is actually used since [7604], but this method has been
introduced to Trac some time after 0.11 release during development for 0.11.3 .
Now code from Trac core is replicated to restore backwards-compatibility with
Trac from 0.11.2.1 down to 0.11 .

This modified version of the change suggested by anonymous reporter of #7231
and addressed in penmark's patch will work better with coming i18n support,
with the drawback, that the implicit default value isn't visible within the
configuration system. At least tried to address this by extending option doc.